Field of the Invention
This invention is directed to phase change material/graphite matrix composites that are widely used in thermal management battery systems, and more particularly to a phase change material/graphite matrix composite material that is compressible and flexible, and that can withstand mechanical stresses without breaking apart or losing thermal contact with cells
Discussion of Related Art
Phase change materials (PCM) are widely used in thermal management systems such as those used in lithium ion (Li-ion) batteries. Al-Hallaj et al. disclose using PCM in thermal management of battery system in U.S. Pat. Nos. 6,468,689 and 6,942,944, herein incorporated by reference. PCM is usually impregnated inside pores of graphite matrix surrounding Li-ion cells. The heat released by Li-ion cells is conducted away by the graphite matrix to the PCM. The PCM absorbs large amount of heat close to its phase transition temperature thereby stabilizing and controlling battery temperature to safe operating limits.
The PCM containing graphite matrix mentioned in the above Patents has a few drawbacks and limitations because of material composition of the graphite matrix. Graphite matrix generally has low electrical resistivity which is not desirable because it can cause electrical short-circuit between battery cells. Graphite matrix composite also generally has poor mechanical characteristics such as has low mechanical yield strength, brittleness, being inflexible, and being a non-compressible material. These mechanical characteristics can be important for Li-ion battery applications, such as for prismatic and pouch cells usage that exert mechanical stress on the graphite matrix composite because of their continuous expansion during charging and discharging cycles. Prismatic cells (or pouch cells) are being widely used in many high power and high energy density applications because they have high energy and power density, but they usually expand and contract during charging and discharging cycles. Thus, there is a continuing need for improvement in the graphite matrices, in both thermal properties of the composite, such as thermal conductivity and latent heat, and for the mechanical properties, such as material strength, flexibility and resiliency.